Paleodemography seeks to discern the demographic parameters of ancient POPULATIONs through archaeological evidence. The foundational assumption in paleodemographic reconstructions is that the age and sex distribution of unearthed mortality samples accurately represent the death rates of the POPULATIONs they belonged to. Under the principle of uniformitarianism, it is assumed the biological processes related to mortality and fertility in human POPULATIONs have remained constant over time. Apart from the importance of overall numbers and rates of POPULATION growth, analyzing POPULATION structure offers deep insights into historical demographic processes and societal behaviors. The structure of a POPULATION in turn is shaped by the aggregate impact of health and disease experiences over time within a POPULATION. In certain cases, demographic data may provide evidence of a specific cause of death. Catastrophic events, whether natural mass disasters or high-mortality disease epidemics yield mortality distributions that reflect the age structure of the POPULATION at risk. Moreover, human migration significantly affects disease distribution globally, impacting indigenous POPULATIONs, while other societal factors like war can also change the POPULATION structures. This paper reviews the determinants that shape and change the demographic structures of ancient POPULATIONs by using the paleo-demographic approach.
Extended Abstract
Introduction
Paleodemography is the study of fertility and mortality rates and POPULATION distribution and density in ancient human POPULATIONs that lack written documents containing evidence of their demographic behavior. Therefore, paleodemography relies on material evidence and archaeological discoveries. One of its main approaches is the estimation of demographic parameters from anthropological data, notably the estimated sex and age of human skeleton remains from the past. Changes in the size, structure, and DYNAMICS can be predicted for a particular POPULATION if fertility, mortality, and migration rates for that POPULATION are known. A stable POPULATION is a POPULATION that is closed to migration and has an unchanging age-sex structure that increases or decreases in size at a constant rate. In a stable POPULATION, theoretically closed to migration with a constant age-sex structure, the number of individuals in each age group increases or decreases at the same rate as the whole POPULATION. Normally, it takes about 50 to 100 years for the age distribution of the POPULATION to achieve a stable structure. However, since no real POPULATION maintains constant fertility and mortality schedules for long periods of time, and few real-life POPULATIONs are truly closed to migration, the concept of a stable POPULATION is largely theoretical. Nonetheless, pre-industrial human POPULATIONs closely approximated this model.
The age structure of a POPULATION refers to the distribution of the number of individuals by chronological age at the time of census. The age-at-death structure of a mortality sample differs from the age structure of the living POPULATION where the death occurred. In most POPULATIONs, the risk of death varies with age. The probability of death varies by age in most POPULATIONs, being higher among the very young and very old people. An increase in fertility and a decrease in mortality change the age structure of a living POPULATION towards a higher proportion of young individuals. However, the effect of changes in fertility is much more specific because it affects the birth cohort, whereas mortality is distributed over all age groups. In POPULATIONs with positive intrinsic growth rates, the ratio of young to elderly exceeds that of a stable POPULATION.
The sex composition, or sex ratio, of a POPULATION is calculated by dividing the number of males by the number of females, and it changes with age due to sex-specific mortality differences. Migration also significantly influences the sex ratio. In contemporary human POPULATIONs, the sex ratio at birth is typically around 1.05. This ratio is influenced by the greater mortality rates in males following birth throughout all age groups, which leads to an equalized sex ratio close to 1.0 by the onset of reproductive age. Female infanticide also affects this ratio.
Data and Methods
This article is the result of a review research, that utilized databases such as Jstor, Google Scholar, Academia, and ResearchGate to gather relevant literature. The search focused on keywords including ‘paleodemography’, ‘sex ratio’, ‘age structure’, and ‘POPULATION structure within ancient POPULATIONs’ as mentioned in the title or keywords. All accessible articles, theses, and books in English were considered, irrespective of publication year, and were freely downloaded. Owing to the nascent stage of paleodemography in Iran, Persian sources were not included in the search.
Findings
The basic pattern of attritional mortality, in which the very young and the very old individuals are at greater risk of death, can be reconstructed for some ancient POPULATIONs. Influential factors in changing POPULATION structure include both cultural and environmental factors. Sedentary POPULATIONs are thought to maintain higher fertility rates than nomadic ones, which is likely reflected in their POPULATION structure, with a relatively larger number of infants and juveniles due to rapid POPULATION growth. While age-specific mortality in hunter-gatherer POPULATIONs follows the expected pattern of all human societies—high infant mortality, low risk of death in the second decade of life, and rising mortality risk with increasing age during adulthood—the age-specific fertility pattern is distinctive. The peak of women's fertility in hunter-gatherer societies occurs in the late twenties to early thirties of their lives, in contrast to many sedentary agriculturalist POPULATIONs, where it peaks in the early twenties. This is despite the fact that the average age of menopause remains constant across all POPULATIONs. For this reason, the peak fertility rate of women in agricultural POPULATIONs is about 0.4-0.5 births per year compared to maximum values of about 0.3 births per year in hunter-gatherer POPULATIONs. Migration also impacts the demographic structure of both origin and destination POPULATIONs. The desire to migrate depends strongly on socio-economic conditions and cultural norms.
Catastrophic mortality, resulting from natural disasters, disease epidemics, or violent inter-group conflicts significantly limits long-term rates of human POPULATION growth. Catastrophic mortality may be underrepresented in the archaeological record, as these events are often inimical to the organized burial of the victims. The age structure of the military organization is typically young adult males with a modal age at death in the early twenties, whereas civilian groups reflect the living POPULATION’s age distribution. Ethnographic data on primitive societies show that many societies targeted noncombatants during the war. There is evidence of variation in age- and sex-specific mortality risk during floods and earthquakes, showing consistently lower mortality rates for young adults and infants compared to children and the elderly, and for men compared to women. Famine induces a demographic crisis characterized by excessive mortality, reduced fertility, and interregional migration. Famine-related deaths are often indirectly caused by increased infectious diseases such as diarrheal illnesses, rather than malnutrition and starvation. A pattern of fertility decline followed by compensatory fertility is a regular feature of famines, attributed to a combination of both biological and social factors. The age-specific mortality pattern in epidemics can be either attritional, catastrophic, or a combination of both.
Conclusion
As discussed, paleodemographic methods can address a wide range of archaeological questions. These include assessing the impact of environmental changes on human POPULATIONs; understanding demographic responses to changes in subsistence technology; examining the rate of POPULATION growth, migration, and cultural changes; exploring the interaction between POPULATION structure and infectious diseases; evaluating the attritional and catastrophic mortality balance in historical POPULATIONs; determining mortality level due to warfare, infanticide, and other interpersonal violence; and investigating the role of POPULATION processes in social conflicts and cultural collapse. Comparative analyses of POPULATIONs indicate that infectious diseases and trauma were the predominant causes of death in the past.